1. Field of the Invention
The present invention relates to image forming apparatuses and control methods thereof.
2. Description of the Related Art
Along with increased high image quality of image forming apparatuses such as copiers in recent years, there is a concern that copiers will be abused. For example, there are image forming apparatuses capable of duplicating items such as securities, which are not intended to be duplicated, with an image quality almost indistinguishable from the original items.
For this reason, various techniques have been proposed for preventing or restraining unlawful duplication of copy-prohibited documents. Japanese Patent Laid-Open No. H04-294682 proposes a technique for forming a decodable tracing pattern, which is difficult to distinguish with the naked eye on a duplicated item, but which is read using a scanner. Information that enables a copier device to be specified is encoded into the tracing pattern. This enables a cue to be obtained for specifying the device used in duplication when a copy-prohibited document has been duplicated.
There are also techniques in which a pattern such as a barcode is appended to a document and outputted. Information for restricting usage is achieved using this pattern, thereby allowing usage of a document to be restricted. An image in which code information is expressed as a pattern image in this manner is referred to as a code pattern image. The aforementioned tracing patterns are also a type of code pattern image.
In this regard, if the minimum element unit of the code pattern image (for instance, the number of dots) can be made smaller, it is possible to reduce the overall size of the code pattern image and to increase the amount of information that is expressible using a single code pattern image. For this reason, there has been a tendency for the minimum element unit to become progressively smaller. Along with this, devices for outputting the code pattern images are required to accurately output the minimum element unit of the code pattern images.
In recent years, copiers have undergone advances in technologies for providing high image quality, and they have become capable of outputting dots of extremely small sizes. However, dot reproducibility gradually decreases in copiers due to changes over time. For example, dots and lines thicken undesirably due to such causes as decreases in the amount of charging because of toner deterioration. In this manner, the required reproducibility cannot be achieved. If a code pattern image is outputted while there is insufficient reproducibility, the information contained in the code pattern image will change undesirably since the fine lines of the pattern thicken undesirably.
Japanese Patent Laid-Open No. 2000-188677 proposes a device in which a code pattern image is not outputted when the resolving power [of the device] has decreased to such an extent that the code pattern image to be outputted would not be able to be read and decoded. Furthermore, Japanese Patent Laid-Open No. 2004-289873 proposes preparing a plurality of LUTs (look-up tables) for code pattern images.
The above-mentioned Japanese Patent Laid-Open No. 2000-188677 prevents outputting a code pattern image that would undesirably result in incorrect information at the time of reading. However, the functionality of outputting code pattern images is lost. On the other hand, Japanese Patent Laid-Open No. 2004-289873 does not extend to carrying out adjustments on the image forming engine. For this reason, when the minimum element unit of the code pattern images becomes smaller, merely selecting an LUT is insufficient as a solution.